Among conventional air-conditioning apparatus such as multi-air-conditioning systems for a building, there exists an air-conditioning apparatus that, by circulating a refrigerant from an outdoor unit to a relay unit and circulating a heat medium such as water from the relay unit to indoor units, realizes a cooling and heating mixed operation by reducing the transport power for a heat medium such as water while circulating the heat medium through indoor units (see Patent Literature 1, for example).
There also exists an air-conditioning apparatus which includes a circuit that performs liquid injection from a high-pressure liquid pipe of the refrigeration cycle into a compressor in order to reduce the discharge temperature of the compressor, and which is able to apply control to keep the discharge temperature to a set temperature regardless of the operating state (see Patent Literature 2, for example).
Furthermore, there also exists an air-conditioning apparatus using R32 as a refrigerant that injects refrigerant from the outlet side of a gas liquid separator installed on a high-pressure liquid pipe of the refrigeration cycle into a compressor whose hermetically sealed container is in a discharge pressure environment (a high-pressure shell compressor) (see Patent Literature 3, for example).